what do the people in your game do with the ships?

James Mullen 
When I was designing my Space Pirate Game (never finished it) I only acknowledged two types of vessels. Fighters (Ships up to about four times the size of the Space Shuttle), and Capital Ships. This was done mainly because a) Space Dogfighting is fun and b) Space Combat is not fun when you don't get to do anything. So all players had Fighters, and were assumed competent. Then I made all the Capital ships basically dungeons which the Pirates boarded.

While I understand the want and need of different classes of ships, you really just need to ask yourself what do the people in your game do with the ships? If your ship is just a moving castle (Like In Battlestar Galatica), then you don't actually need a lot of details for opposing ships. What you need is reasons that they would avoid each other or get in conflict with each other, and then a system in which they would avoid conflict.

In less cinematic universes (Say, Honor Harrington's series), combat is almost over before people even start it. Computers calculate everything, fire, and then calculate the next results it needs to launch. That means the emphasis of the role playing is disaster management and negotiations, with some tactical roles thrown in. Class of ships is less necessary as lethality of ships. As our history has shown, even very small ships can cripple large ones.